HIV-1 infected individuals who are injecting opioid drugs show increased cognitive defects and undergo an accelerated rate of progression to AIDS. Accumulating evidence suggests that opioid drug abuse intrinsically exacerbates the pathogenesis of HIV-1. We have found that neuronal death is preceded by a prolonged period of synaptic culling, functional losses, and dendritic pathology that are presumed reversible. Importantly, opioid abuse potentiates the neuropathogenesis of HIV-1 by synergistically increasing dendritic pathology (varicosity formation, beading, fragmentation, pruning), while promoting additive dendritic spine losses (plasticity). This has been verified in medium spiny neurons (MSN) of the striatum and synaptic pruning has been confirmed electron microscopically. Moreover, behavioral defects in locomotor activity are accompanied by synaptic losses and dendritic pathology in the absence of demonstrable neuron death, suggesting that sublethal neuronal injury and reduced synaptic connectivity underlie the ability of opioids to aggravate HIV-1-associated neurological disorders (HAND). While death per se is significant, the interruption of events preceding neuron death may be more strategic therapeutically. This grant will focus on the functional level of MSN by investigating the underlying physiological mechanisms of opioid +/- HIV-induced excitotoxicity. It is hypothesized that Tat induces changes in the cellular homeostasis and excitability of MSN that are exacerbated by opioid drugs through a complex sequence of events involving OR-mediated pathways. In vitro approaches are being proposed by assessing the effects of opioid drug and HIV-1 Tat-induced neurotoxicity in dissociated cortical-striatal cell cultures. Whole-cell patch-clamp recordings wil be conducted in voltage- and current-clamp mode by assessing action potentials as well as sodium, potassium, AMPA, NMDA, and calcium (Ca2+) currents. The role of OR will be elucidated by applying pharmacological (OR antagonists), genetic (OR knockout mice) and silencing (silencing NMDAR) strategies to identify mechanisms underlying opioid + HIV protein interaction. To sort out whether opioids exacerbate the excitotoxic effects of Tat in the striatum via OR on MSN we will conduct experiments in vivo using two types of Cre-lox mice. Conditionally deleting OR at key sites will define the targets and associated mechanisms by which opioids exacerbate neuronal excitability (action potentials, ion channel activity, ion imaging, mitochondrial membrane potential), injury (including dendritic pathology and spine density), and behavioral defects (locomotor activity) in the striatum.